With recent development of communication systems, mobile electronic devices such as notebook computers and mobile phones have been rapidly spread. While mobile electronic devices are advanced in terms of high functionality, they are also advanced in terms of diversification of function and shape. Then, secondary batteries serving as power sources therefor have been increasingly demanded for compact size, lightweight, high energy density, and the like.
For the purpose of obtaining a secondary battery with lightweight and a large energy density, a secondary battery in which a sulfur compound and an organic compound are used for an electrode active material has been developed. Patent Literatures 1 (U.S. Pat. No. 4,833,048) and 2 (JP2715778B) disclose a secondary battery in which an organic compound with a disulfide bond is used for a positive electrode. In such a secondary battery, an electrochemical redox reaction along with formation and dissociation of a disulfide bond is utilized. The secondary battery described in Patent Literatures 1 and 2 is constructed of an electrode material including an element with a low specific gravity, such as sulfur or carbon, as a main component, and exerts a certain effect in terms of a secondary battery with a high energy density.
However, the secondary batteries in Patent Literatures 1 and 2 could not sometimes perform a stable charge-discharge cycle because the dissociated disulfide bond is allowed to be again bonded at a low efficiency and active materials in electrodes are diffused in an electrolytic solution. Thus, in some cases, there has been a disadvantage in that a charge-discharge cycle is repeated to result in the reduction in capacity with ease.
As a secondary battery utilizing an organic compound, a secondary battery using a conductive polymer for an electrode material has also been proposed. In this secondary battery, the doping and dedoping reaction of a conductive polymer with an electrolytic ion is utilized. The doping reaction refers to a reaction for stabilizing a charged radical generated by oxidization or reduction of the conductive polymer by a counter ion.
Patent Literature 3 (U.S. Pat. No. 4,442,187) discloses a secondary battery in which such a conductive polymer is used for a positive electrode or negative electrode material. The secondary battery in Patent Literature 3 is constructed of only an element with a low specific gravity, such as carbon or nitrogen, and has been expected as a secondary battery with a high capacity.
However, the conductive polymer has such characteristics that a charged radical generated by redox is delocalized over a wide range of a π electron conjugated system, and interacts with other charged radical to cause electrostatic repulsion or radical dissipation. This allows the charged radical generated, namely, the doping concentration to be limited, and restricts the capacity of the secondary battery. For example, it has been reported that the doping rate of a secondary battery in which polyaniline is used for a positive electrode is 50% or less, and the doping rate in the case of polyacetylene is 7%. Although the secondary battery in which the conductive polymer is used for an electrode material exerts a certain effect in terms of weight saving, a secondary battery with a large energy density has not been obtained.
On the other hand, as a secondary battery in which an organic compound is used for an electrode active material, one has been proposed in which the redox reaction of a radical compound is used. This secondary battery is referred to as an organic radical battery. Patent Literature 4 (JP2002-151084A) discloses an organic radical compound, such as a nitroxyl radical compound, an aryloxy radical compound, and a polymer with a particular aminotriazine structure, as an electrode active material, and a secondary battery using the organic radical compound as a positive electrode or negative electrode material.
Patent Literature 5 (JP2002-304996A) discloses a secondary battery using in particular a compound with a cyclic nitroxyl structure, among nitroxyl compounds, as an electrode active material. The cyclic nitroxyl structure is known to exhibit stable p-type redox. As a polyradical compound for use as an electrode active material, a nitroxyl radical compound such as poly(2,2,6,6-tetramethylpiperidine-1-oxyl methacrylate) (PTMA) including 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is known.
Patent Literature 6 (JP2008-280400A) discloses a secondary battery using a stable aryloxy radical compound as an electrode active material. This compound is known to exhibit n-type redox.